1. Field of the Invention
The preferred embodiments are directed to a method of imaging a sample using a scanning probe microscope (SPM), and more particularly, a method for automatically recognizing and verifying small-scale sample features, such as nano-asperities, with the SPM at high image resolution and scan rates.
2. Discussion of the Prior Art
A scanning probe microscope, such as an atomic force microscope (AFM) operates by providing relative scanning movement between a measuring probe and a sample while measuring one or more properties of the sample. A typical AFM system is shown schematically in FIG. 1. An AFM 10 employing a probe device 12 including a probe 14 having a cantilever 15 is shown. A scanner 24 generates relative motion between the probe 14 and a sample 22 while the probe-sample interaction is measured. In this way, images or other measurements of the sample can be obtained. Scanner 24 is typically comprised of one or more actuators that usually generate motion in three orthogonal directions (XYZ). Often, scanner 24 is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner 24 may be an assembly of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY scanner that moves the sample and a separate Z-actuator that moves the probe.
In a common configuration, probe 14 is often coupled to an oscillating actuator or drive 16 that is used to drive probe 14 at or near a resonant frequency of cantilever 15. Alternative arrangements measure the deflection, torsion, or other motion of cantilever 15. Probe 14 is often a microfabricated cantilever with an integrated tip 17.
Commonly, an electronic signal is applied from an AC signal source 18 under control of an SPM controller 20 to cause actuator 16 (or alternatively scanner 24) to drive the probe 14 to oscillate. The probe-sample interaction is typically controlled via feedback by controller 20. Notably, the actuator 16 may be coupled to the scanner 24 and probe 14 but may be formed integrally with the cantilever 15 of probe 14 as part of a self-actuated cantilever/probe.
Often a selected probe 14 is oscillated and brought into contact with sample 22 as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe 14, as described above. In this regard, a deflection detection apparatus 25 is typically employed to direct a beam towards the backside of probe 14, the beam then being reflected towards a detector 26, such as a four quadrant photodetector. Note that the sensing light source of apparatus 25 is typically a laser, often a visible or infrared laser diode. The sensing light beam can also be generated by other light sources, for example a He—Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector 26, appropriate signals are transmitted to controller 20, which processes the signals to determine changes in the oscillation of probe 14. In general, controller 20 generates control signals to maintain a relative constant interaction between the tip and sample (or deflection of the lever 15), typically to maintain a setpoint characteristic of the oscillation of probe 14. For example, controller 20 is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.
A workstation is also provided, in the controller 20 and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM. Notably, scanner 24 often comprises a piezoelectric stack (often referred to herein as a “piezo stack”) or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Additionally, flexures are used to increase the stiffness of actuator in one or more axis, as described in copending application Ser. No. 11/687,304, filed Mar. 16, 2007, entitled “Fast-Scanning SPM Scanner and Method of Operating Same.” Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.
As the utility of SPM continues to develop, a need has arisen for imaging different types of samples at greater speeds to improve sample measurement throughput, including imaging larger sample areas. Although AFM intrinsically has a resolution determined by the probe apex, usually a few nanometers, the level of detail in the AFM image depends on the scan size. For example, a conventional 512×512 pixel image has detail of 2 nm if the scan size is 1 μm but 20 nm and 200 nm per pixel if the scan size is 10 μm and 100 μm respectively. Clearly, high resolution and large scan size come at a cost of throughput. Using the same example, if the 10 μm image size requires 2 nm detail, the pixel density needs to be 5000×5000, instead of 512×512. Because AFM uses faster scanning to acquire images, increasing data in each scan line by 10 times will require the Z feedback loop to be 10 times faster to obtain of the data for each pixel. The scan time will also be ten times longer to obtain an image because the 5000 lines of data are obtained in sequence, increasing the time to obtain a normal image from 8 min per frame to more than 1 hour per frame.
Other factors can limit imaging speed as well, including the cantilever response time, the usable scanner bandwidth in X, Y and Z directions, the slew rate and bandwidth of the high voltage amplifier that drives the scanner, the speed of cantilever force sensing, as well as the demodulation system and the tracking force feedback system.
SPM images are typically constructed of arrays of measurements recorded at different locations on the sample. For example, an image may contain the local value of the relative sample height measured over an array of different XY locations on the sample. Alternative measurements can include amplitude, phase and frequency response of the cantilever, as well as electric and magnetic forces, friction, and stiffness of the sample, etc. The measured data is representative of the sample surface.
In addition to the speed constraints noted above, high resolution imaging in a large area sample is usually achieved progressively. In particular, a survey scan in a large area is often used to determine if sample includes an interesting feature. If the feature is identified, the AFM will allow the user to zoom in on the feature multiple times until the desired resolution is attained or the limit of the tip radius is reached. The judgment of whether a feature should be further imaged (higher resolution) is provided by a trained operator and the zoom-in scan can be manually initiated with most AFM tools.
A specific application of nanometer feature detection and measurement is nano-asperity measurement of disks used in data storage. Nano-asperities are concave features ranging a few nano-meters in height and 20-40 nm in diameter on hard disk media. During a data read/write process, the distance of the magnetic pole tip of the read/write head to the disk media is also in the range of nanometers. A nano-asperity may permanently damage or “crash” the disk read/write head if its height exceeds the fly height of the read/write heads. As a result the disk media are routinely inspected to monitor the occurrence of the nano-asperities, preferably using an AFM.
One problem in this regard is that the disk area to be analyzed is relatively large when considering the size of the defects that are intended to be identified. Optical techniques are able to measure large areas in a relatively short amount of time; however, such techniques are not able to identify nano-asperities. AFM provides the ideal solution in this regard. The trade off, however, is that an AFM scan takes a relatively long time with a scan speed in a range of about 1 Hz, such that bringing attention to image at high data density locations in a 10 micron scan size becomes prohibitively time consuming.
Moreover, a related problem is that, with a 1-2 nanometer height of the nano-asperities, precision greater than 1 Angstom is required. To achieve this level of precision, the AFM must be operated at a relatively slow rate to yield usable data. A compromise used in current practice is to scan a 10 μm×10 μm area for relatively large sampling coverage at 512×512 lines per image, which takes about 8.5 minutes to complete one image, and over an hour to survey a small portion of the sample, e.g., a selected area of disk. Even when imaging at this carefully slow rate, however, the pixel size of each data point is about 20 nm, which is similar in size to the nano-asperities. Therefore, not only is it possible that the measurement easily misses small asperities, but the height data, when the pixel does correspond to a nano-asperity, barely reflects the true height and shape of the concave asperity with a single, or even a few, data points.
Using AFM also presents difficulty when attempting to correctly differentiate and identify the characteristics of nano-asperities, especially considering system noise. System noise can be caused by a large and diverse number of sources, such as the actuators, the probe cantilever 15, electrical signal noise, etc. System noise can cause false positive identification of a nano-asperity, mischaracterization of a nano-asperity, etc. Noise is incoherent. As a result, when features are larger they typically can be readily distinguished by a proficient user. However, as feature size gets smaller, this property of noise becomes less distinguishable. During slow speed, high resolution scanning, noise can be accounted for using data averaging at slow speeds, such as approximately 1 Hz. However, this technique is not applicable during higher speed scans where the number of data points associated with a point of interest is much lower and would tend to distort rather than enhance detected features, and still relies on a trained operator.
Moreover, there are disadvantages to relying on human judgment, however, even when the user is a trained operator. This is especially the case when the feature is very small and the image is relatively noisy. Referring to FIG. 2, a schematic sample AFM image 27 produced using standard AFM imaging techniques is shown, according to an exemplary embodiment. Image 27 illustrates the output that is generated by the AFM 10 including an identification of a nano-asperity 28, along with a plurality of noise instances 29. As understood in the art, when attempting to identify and image sample features 28 on the scale of nano-asperities, such features of interest are indistinguishable from the noise instances 29 through normal human operator perception. As a result, using AFM to identify and image nano-asperities has not been prolific.
With the continuing trend of reducing the fly height of read/write heads, the importance of qualifying disks for smaller asperities continues to increase. The art of small-scale defect detection is therefore in need of a technique of operating an AFM at a faster rate, while still being able to provide high resolution data as well as sufficient data quality control.